The present invention relates to a technique for depositing a platinum film, which is used as a bottom electrode of a capacitor in a high density DRAM and in a non-volatile memory cell including a FRAM memory cell. More particularly, the invention relates to a technique of forming a platinum film which can prohibit oxidation of a functional intermediate film (e.g., such as a diffusion barrier layer of a high density DRAM) which is provided below the platinum film.
In this specification, the term "functional intermediate film" is used to cover all of the films used in electronic devices for prohibiting inter-diffusion of silicon in a polysilicon layer and platinum in a platinum electrode, for electric connection or insulating, and/or for enhancing adhesion between a platinum film and a substrate. As well-known in the art, a functional intermediate film may be referred to as a "diffusion barrier layer," a "conductive plug layer," an "adhesion layer," or an "insulation layer" in accordance with its function. Due to the presence of these layers, some problems can arise.
For example, a platinum film is usually used as a bottom electrode in a capacitor of a high density DRAM cell or of a non-volatile ferroelectric memory device. In this regard, when the platinum film is served as a bottom electrode of a capacitor in a DRAM cell and a transistor is connected to the capacitor via a conductive plug formed from polysilicon, platinum silicide is formed on the interface between the polysilicon layer and the platinum film during the platinum depositing, subsequent annealing and/or the other post-processing, since the platinum film directly contacts the polysilicon layer. To avoid this phenomenon, a diffusion barrier layer formed from a nitride (e.g., such as TiN, Ti--Si--N and GaN) is formed between the platinum film and the polysilicon.
Even if a diffusion barrier layer is formed, oxygen gas introduced during the post-annealing or high-dielectric/ferroelectric oxide layer depositing process can diffuse through voids formed between grain boundaries in the platinum film. Because the grains of the platinum film deposited by conventional processes have vertical columnar structures with inter-columnar voids, oxygen introduced from the above-mentioned process can easily diffuse through the platinum film to the diffusion barrier layer. The oxygen gas diffused through the platinum film then oxidizes the diffusion barrier layer and forms an oxidized insulation layer such as TiO.sub.2 and Ta.sub.2 O.sub.5 between the barrier layer and the platinum film. Consequently, the function of the platinum film as an electrode can become deteriorated or even lost. In particular, if the diffusion barrier layer is formed from TiN, N.sub.2 gas is produced while an oxidized film of TiO.sub.2 is formed on the surface of TiN layer, and the N.sub.2 gas can cause the platinum to expand and become released from the barrier layer. This phenomenon is known as "buckling."
In a DRAM device or a non-volatile memory device or various types of sensor devices, a functional intermediate film, more particularly a conductive adhesion layer, formed from Ti, Ta, TiN, TiW or W also can be interposed between the substrate (or a insulation layer formed on the substrate) and the platinum film in order to increase adhesion strength therebetween. In this case, although a ferroelectric oxide film is deposited after the adhesion layer and the platinum film have been formed, oxygen gas and/or adhesion layer material can diffuse through the platinum film formed by conventional methods, thereby forming an insulation layer such as TiO.sub.2 and Ta.sub.2 O.sub.5 in a DRAM cell due to the oxidizing of the adhesion layer. Moreover, "buckling" of the platinum film may occur if TiN was used, since N.sub.2 is generated due to the oxidation of TiN to TiO.sub.2. Therefore, the performance properties of a DRAM cell or a non-volatile ferroelectric cell or other device can be extremely deteriorated due to problems caused by oxygen diffusing through the platinum film and resulting in oxidation.
As known in the art, if the platinum film employed as a bottom electrode has (200) preferred orientation, the ferroelectric oxide film which is formed on the platinum film tends to be oriented mostly to one crystallographic direction, preferably to the c-axis direction. Due to this controlled orientation, it has been found that the electrical properties of the electronic device can be highly improved, while its fatigue tendency can be reduced and adhesion strength improved. Therefore, the orientation control of platinum film can be very important.
In addition, platinum films formed by the conventional methods using sputtering often are not dense enough and may have a number of pinholes, pores or hillocks, which may result in device performance problems.
It is seen from the above that methods for forming platinum films that can prevent oxidation of a functional intermediate film (such as a diffusion barrier layer, an adhesion layer, an insulation layer, and a conductive plug layer) are desirable. It is also desirable to be able to control the orientation of such platinum films and have defect-free platinum films.